1. Field of the Invention
The present invention relates to the general fields of manufacturing process control and artificial intelligence, and more particularly, to an automated system for developing a process cycle for composite parts and controlling that cycle based on the desired outcome of the process cycle in terms of processed materials properties.
2. Brief Description of the Prior Art
Composite materials consist of fibers imbedded in matrices which can be made of a variety of materials. The mixture of these components, if properly controlled and fabricated, offers improvements in properties which cannot be realized with most homogeneous materials because of the synergistic effect of the components of the mixture. The fibers offer strength and stiffness to organic matrix composites while the matrix provides a means of preventing buckling of the thin fibers by transferring load between them.
Another advantage of these materials is the tailorability of composite properties. The fibers can be oriented within a part to achieve strength in the most advantageous directions, decreasing the weight of materials needed for a specific requirement.
In order to realize the advantages of composites; however, extremely complex and controlled processing requirements must be met. Not only do the constituent materials have to be processed to each material's specific performance requirements, but the mixture then has to be fabricated from them which has precisely controlled ratios of fiber to matrix and the fibers must be oriented in such a manner as required to meet designed loads. In some processes, such as autoclave curing of thermosetting polymeric matrix composites, the development of matrix mechanical properties is achieved in the same process which determines fiber/matrix ratio and net part shape.
Most fabrication methods for composites involve the addition of heat or energy. One purpose of this heat is to initiate and sustain chemical reactions in thermosetting matrix polymers. Another purpose is to melt or reduce the resistance to flow of the matrix so that the composite can be formed into a shape and compacted into a final desired thickness. In some materials, a portion of the matrix present in the initial mixture may be pressed or bled out to achieve the final desired ratio of fiber to matrix. Whenever heat is applied, the danger exists that volatile components of the matrix, contaminants to the material, or products of chemical reactions may be vaporized and form bubbles within the matrix. Such bubbles, if not removed or suppressed, can result in porosity in the matrix which reduces mechanical properties.
Many composites are presently fabricated by the lamination of layers of premixed, or prepregged, fabric or unidirectional fibers impregnated with matrix, called prepreg. These layers are cut and stacked and then subjected to pressure and heat to form parts. In addition to the complexity represented by control of the fiber/matrix ratio, and prevention of porosity and the curing chemical reaction, the layers of the composite must all achieve relatively uniform properties in order to reduce localized deviations from the desired properties which might have a weakening effect on the entire part. Because of this, thermal and pressure gradients within a part and throughout a batch of parts must be controlled.
As an example of a typical process for fabrication of composites, the autoclave cure of polymeric matrix composites involves the stacking of layers of fabric or unidirectional fiber tows preimpregnated with controlled proportions of unreacted thermosetting polymer on a tool or mandrel. This stack is then placed in a sealed bag connected to a vacuum (and sometimes pressure) source and the bagged part is placed in an autoclave, which is a pressure vessel which can be heated. The autoclave temperature and pressure, and the bag vacuum and pressure are then manipulated with respect to a predetermined timing cycle. These cycles are generally established by modification of cycles which have been successfully used for similar parts and materials.
The above-mentioned means of developing process cycles have been improved somewhat by the incorporation of analytical models which serve to simulate the autoclave process. Such simulations can partially replace the more expensive trials on the autoclave when a full complement of data exists for a material and the processing vessel and tooling and bagging materials. This can reduce the number of trial parts considerably.
Typical current process cycles treat the various requirements of the composite process in sequence. Removal of volatile contaminants and entrapped air is first achieved by manipulation of vacuum combined with slightly increased temperatures, a process step known as debulking. The laminate is then compacted by manipulation of autoclave pressure and temperature to squeeze out excess matrix and press layers of prepreg together. This is done at a slightly elevated temperature which is sufficient to allow flow of the matrix but not high enough to initiate rapid curing. Finally, the temperature is raised to initiate rapid reaction and crosslinking of the polymeric matrix. Pressure may be applied within the bag at this point to suppress volatilization of contaminants or reaction products, generally water.
Such empirical reiterative approaches to cure cycle development are costly and time consuming and may not lead to true optimization, especially if, as has been the case, the process goals are treated in the sequential fashion outlined above. Specifically, autoclave trials require multiple manufacturing runs and extensive testing and qualification of new composite components. This cost can be reduced by use of analytical models to reduce the number of reiterations in the autoclave but these models require expensive generation of data on mechanical, thermal, viscous and diffusive properties of all materials used in the composite, bagging and tooling as well as autoclave characteristics. Even within the same materials, some of these properties may vary significantly, depending on quality assurance and handling history. Control of the above-mentioned autoclave cure is currently based on following within predetermined allowables, the predetermined cycles of temperature, pressure, and vacuum described above. The most advanced controllers are feed-back controllers which manipulate the autoclave temperature to maintain part temperatures, rates of heating and thermal gradients within predetermined constraints. Advanced controllers of this type may use matrix properties such as dielectric constants or ultrasonically measured modulus to determine flowability and compressability so that pressure can be controlled to take advantage of those properties in achieving the desired fiber/matrix ratio and suppressing bubble formation.
These cascade control systems are based on algorithms which correlate a limited number of predetermined factors. In the event of process events which fall outside controller limitations or fail to meet predetermined specification alarm conditions are triggered, and operator intervention is required to adjust or abort the cycle in progress.
U.S. patents of interest include a method U.S. Pat. No. 4,555,268 and a related system U.S. Pat. No. 4,515,545 to Hinrichs et al for controlling a curing process by measuring several parameters, comparing the values with predetermined values, and using the result of the comparisons to control the application of temperature and pressure in an autoclave. A system U.S. Pat. No. 4,399,100 and a related method U.S. Pat. No. 4,496,697 to Zsolnay et al describe a closed-loop automatic process control technique in which capacitance is measured to determine when to apply pressure.
In view of the complexity of the autoclave cure process and the interaction of multiple variables which are beyond direct control of the controller, two needs have been established in the industry. First, there has been a need for an improved means of optimizing cure cycles based on the extensive knowledge that has been developed in the process of developing and using analytical models to simulate the process. This would decrease the cost and learning and qualification time associated with the introduction of new, improved materials as well as the development costs of extending composites to new applications. It would also result in true optimization of the cure cycles which translates into reduced time and energy for processing.
Second, there has been a need to further automate the process of manufacturing composites by introduction of a more flexible controller which is capable of recognizing and intelligently responding to a greater variety of process behavior during the process. This capability offers improved reliability without tightening of precursor material specifications. Such tightened material specifications are costly because of the degree of process and handling controls required an the verification tests necessary to assure quality.
In summary, the need has been noted in the industry to increase automation of the processing of advanced composites by automating the development of cure schedules and improving the flexibility of response of in-process controllers.